Tag Archives: structure

Floating Offshore Structures – Offshore Structure Series

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There are many oil and gas discoveries which are out of reach of fixed structures for one reason or another. They may be in extremely deep water, or the oil or gas deposit might be too small or too widely spread to warrant the high cost of building a fixed structure. In these cases, seabed-completed wells may be connected to a floating platform moored above the field, using a production marine riser. The limiting conditions for fixed installations are not clearly defined, and they have been used in some cases for depths of over 250 meters, although this is in a benign environment. Floating platforms can also be used as the basis for an Early Production System (EPS), in which the appraisal wells drilled from a floating drilling vessel are completed at the seabed and produced to a floating platform carrying the required process plant and other facilities. This allows production to begin and create income whilst a fixed platform is being designed and installed for full field development. In this article, there are some discussions about three main types of floating offshore structures which are Tension Leg Platforms, SPAR and FPSO.

Tension Leg Platforms (Tethered Buoyant Structures)

One more form of offshore platform is what is known as the Tension Leg Platform, or Tethered Buoyant Structure. This method is intended for oil and gas production from water depths of over 500 meters. The platform works in much the same way as a taut moored buoy, which is anchored to the seabed using a vertical wire. The Tethered Buoyant Structure is basically a large, semi-submersible floating vessel, which uses a heavy gravity anchor to moor it to the seabed. Tension force is maintained in these vertical cables by adjusting the buoyancy of the floating platform, to ensure positive tension at all times. This method reduced marine response in the platform to effectively zero in vertical terms, and very little in horizontal terms. Horizontal drift can be further reduced as necessary. By using buoyance against a tension mooring system, this allows the use of a semi-submersible floating platform which can carry an additional load, balancing this out by increasing the buoyancy.

This type of structure is still under development, and there are still many problem points to iron out. How widespread it will become in the future is largely dependent on solving these issues, along with a thorough economic assessment comparing the system to other available ones.

Figure 1 – Tension Leg Platform

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Summary of Comparison between Piled Offshore Platform Structures VS Concrete Gravity Structures

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We’ve discussed about two type of fixed platform structures which are piled offshore platforms and concrete gravity structures. In this article, it summarize the comparison between these two fixed offshore structures.

Figure 1 – Piled Structure and Gravity Structure of Brent (Courtesy of Shell UK)

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Concrete Gravity Structures – Offshore Structure Series

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As the name suggests, the concrete gravity structure is reliant on its own weight, and the capability of the seabed to maintain that weight, in order to remain stable. They are designed particularly with storm conditions in mind. Like other types of structure, they come in multiple design variations, and may be made out of concrete, steel, or a combination of the two. Concrete gravity structures were first used in the Ekofisk Field off Norway, although the design principle had previously been used in lighthouse construction. The Ekofisk structure, which had originally only been intended for oil storage but was then modified for use as a large gas handling and compression plant, was soon followed up with the construction of multiple additional drilling and production concrete gravity structures made from reinforced concrete. Given the huge demand placed on onshore prefabrication sites, and the significance of the water depths available to constructors for fabricating and towing these structures near to the shore, there has been a wide variety of different gravity designs, despite being constrained by the conditions of the construction site. It has been impossible to create an optimized design which is suitable to be built at all available sites.

The concrete gravity structure is built in a tapered shape, with as much of the mass and bulk concentrated as close as possible to the seabed. Ideally, the platform is constructed close to the shore, and the topside facilities are placed in a sheltered site before the offshore tow begins. Then, the whole thing is moved to its final location through the use of ocean-going tugs. This is done as much as possible using a multi-celled caisson raft, which can measure up to 100 meters high and 60 meters wide. From this raft base, a number of columns will be carried up to the full height of the structure. When the raft reaches the offshore location, the caisson is water ballasted and landed on the sea bed, Offshore installation can therefore take as little as a few days, which is certainly an advantage in harsh areas which have short fair weather periods. Concrete gravity structures can be used in water depths up to 160 meters and with weights of over 300,000 tonnes.

Examples of concrete gravity structures – Ninian Central Platform

Figure 1 demonstrates Ninian Central platform, a large concrete tower with a series of tanks around the base. These concrete fixed platforms are able to store fluids, and can also be attached to export lines, which gives them a significant advantage over steel jacket platforms. Jacket platforms generally lack tanks, although they can be built on deck, which means that their export can be entirely lost if a tanker does not stick to its strict schedule. Concrete platforms also do not need to be secured to the seabed. Thanks to skirts around the concrete, erosion is prevented. Concrete platforms perform exactly the same function as steel jacket platforms, with only the support structure being different.

Figure 1 – Ninian Central platform

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Piled Offshore Platform Structures – Offshore Structure Series

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The technology used for offshore oil and gas production has always needed to be flexible and fast-developing, in order to meet the wide range of challenges that different environments can present. Overall, the most important requirement of this technology has been a working deck which is mounted on a larger structure that provides enough space for all necessary production equipment, like processing facilities to separate oil, gas, and water, as well as pumps, compressors, connections, and living space for workers on the rig.

When platform development was not so advanced, the well drilling would usually be completed before the production process began, to ensure the safety of workers. Additional equipment and accommodation would also usually be located on a separate structure for the same reason. However, as wells were constructed in ever deeper water, new types of platforms needed to be designed.

Deep-water structures are very high-cost, and it is therefore more economically viable to accommodate workers and equipment on a single platform. Coupled with improved safety practices and fire prevention, this means that is now commonplace for offshore development to be situated on just one structure.

The most fundamental requirement for offshore drilling is a platform from which the whole operation can be run. In most cases, this is done from a fixed platform, but in recent decades floating production facilities have been successfully developed, and these are becoming increasingly common. These floating production units are the most commonly used when it comes to deep-sea applications.

With fixed platforms, there are two basic types, both with a subset of variations for specific purposes. These two types are piled structures and concrete gravity structures (Figure 1). Neither has gained precedence over the other, mainly because of frequent changes in the cost of materials, equipment, and specialized labor needed to construct them, as well as changing demands from the offshore industry as to the size of the platform.

Figure 1 – Piled Structure and Gravity Structure of Brent (Courtesy of Shell UK)

Each type has its own disadvantaged. Piled structures can be lengthy and costly to build, and during this period they are susceptible to damage from bad weather. They also lack oil storage capacity, and drilling and processing facilities and living space must be constructed only when the base structure has been fully completed. On the other hand, concrete gravity structures are more expensive to put together, and once the foundation has been put down, they are difficult to modify, meaning if the soil conditions change even slightly from those anticipated, there could be problems with the structure that are highly expensive to put right. In this article, it will describe basic details of piled offshore structure. Continue reading →

5 Largest Offshore Structures (Rig and Production Platform) in The World

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Offshore structures (rig and production platform) are complex facilities to drill wells (options) and produce gas from wells from offshore locations. This is one of the most fascinating structures in the world and this article will show 5 largest offshore structures on the planet.

1. Berkut Platform

Berkut Platform

Berkut Platform (Courtesy of Rosneft)

Berkut is the world’s biggest oil platform which has begun commercial production at the Sakhalin-1 offshore project in Russia’s Far East. The Berkut oil rig is expected to extract 4.5 million tons of oil annually. The Sakhalin-1 Consortium was formed in 1996 is the first major shelf project in Russia created under terms of a Product Sharing Agreement (PSA). The international consortium is made up of the US major ExxonMobil (30 percent), Japan’s Sodeco (30 percent), Russia’s Rosneft (20 percent) and India’s ONGC Videsh (20 percent). The Berkut platform is expected to produce 12,000 tons of oil daily or about 4.5 million tons annually, raising the total output of the Sakhalin-1 Consortium to 27,000 tons a day. Continue reading →